I bought this Ultrathin 20000mAh Portable Power Bank (5.3 oz./151 g including USB-to-µUSB cable) on eBay. It's noticeably lighter than the Anker PowerCore 10000, which at 6.35 oz./180 g was considered the "go-to" power bank among through-hikers just two years ago. It's way lighter than the similar-capacity Anker PowerCore 20100 at 12.56 oz./356 g. And it's cheap: under $9 shipped.

My unit came in silver; they also make them in blue, pink, black, and gold (randomly distributed). The housing appears to be aluminum. Given the low weight and high capacity I'd guess they're using lithium ion cells. There is no documentation at all, so I fiddled with it a bit to deduce the how-to.

There is one button, between "ON" and "OFF" labels. When I pressed it 4 small blue LEDs came on to indicate that it was about 4/4 charged (presumably 17,500+ mAh). There's a µUSB port labeled "IN". I plugged the included cable into that port, with the other end in a USB port in my PC. Immediately the first 3 blue LEDs lit up steadily, and the rightmost one started blinking to indicate that it was charging the last 1/4 of its capacity. There are two USB outputs, one labeled "OUT 1A" and the other labeled "OUT 2.1A"; I should be able to charge my phone and headlamp simultaneously. I don't know if this power bank supports powering other devices while it's being charged, but that's not a big concern because I've got a 4 USB port AC adapter so I won't need to do any daisy-chaining when I've got electrical outlet access.

There's no mention of "Quick Charge" or other such technology, so presumably charging won't be fast. Of the three prime metrics of backpacking technology (Light, Cheap, Good/Fast) I'm happy with the two I got here.